An information equipment such as a computer or a communication equipment unintentionally radiates unwanted electromagnetic waves, which have been produced from electric signals generated inside the information equipment or an interface connecting the information equipment to another. The unwanted electromagnetic waves contain information processed inside the information equipment. If a kind of the information interface can be clarified, we can reproduce the information by receiving the unwanted electromagnetic waves.
FIG. 1 is a view showing an example of a circuit for generating analog R, G, and B signals that are typical image signals processed with a computer.
The analog RGB signal generation circuit 100 shown in FIG. 1 has a vertical synchronization signal generator 101 to generate a vertical synchronization signal, an N-times frequency multiplier 102 to multiply the frequency of the vertical synchronization signal from the vertical synchronization signal generator 101 by N, a horizontal synchronization signal generator 103 to generate a horizontal synchronization signal based on the signal from the N-times frequency multiplier 102, an N-times frequency multiplier 104 to multiply the frequency of the horizontal synchronization signal from the horizontal synchronization signal generator 103 by N, a dot clock signal generator 105 to generate a dot clock signal based on the signal from the N-times frequency multiplier 104, a video RAM 106 to store R, G, and B image data, and analog modulators 107, 108, and 109 to modulate dot clock signals generated by the dot clock signal generator 105 according to R, G, and B (red, green, and blue) image data from the video RAM 106.
The RGB signal generation circuit 100 outputs the vertical synchronization signal V, horizontal synchronization signal H, and R, G, and B signals, as shown in FIG. 2. FIG. 2 shows examples of the vertical synchronization signal V, horizontal synchronization signal H, and R, G, and B signals. A signal to be displayed as image information is, as shown in FIG. 2, an analog signal representative of the brightness levels of dots, and each period of a brightness string corresponds to each period of the dot clock signal.
There are a variety of image resolution standards such as CGA, EGA, VGA, SVGA, QVGA, XGA, WXGA, SXGA, and UXGA. An image information transmission electric signal based on such a standard has periodicity and includes the above-mentioned synchronization signals. Accordingly, by utilizing the periodicity, a third person can illicitly steal image information from electromagnetic waves produced from the image information transmission electric signal.
FIG. 3 is a view showing an example of a reproduction apparatus used to illicitly steal image information. The reproduction apparatus 200 shown in FIG. 3 has an antenna 201, an amplifier/attenuator (AMP/ATT) 202 to, for example, amplify a signal from the antenna 201, a receiver 203 to carry out a filtering process and the like, an amplifier 204 to amplify a signal from the receiver 203, a limiter 205 to limit the amplitude of a signal from the amplifier 204, a vertical synchronization signal generator 207 to generate a vertical synchronization signal, a horizontal synchronization signal generator 208 to generate a horizontal synchronization signal, and a display to display R, G, and B signals from the limiter 205 in synchronization with the vertical synchronization signal and horizontal synchronization signal. The antenna 201 receives electromagnetic waves that are spontaneously generated by, for example, an interface line passing an image information signal. The reproduction apparatus 200 reproduces the image information signal from the received electromagnetic waves, thereby allowing a third person to unjustly steal image information.
FIG. 4 is a view showing another example of a reproduction apparatus. The reproduction apparatus 200A shown in FIG. 4 employs, instead of the limiter 205, display 206, vertical synchronization signal generator 207, and horizontal synchronization signal generator 208 of the reproduction apparatus 200 shown in FIG. 3, an AD converter 209 and a notebook personal computer 210 after an amplifier 204. In the reproduction apparatus 200A of this configuration, software inside the notebook personal computer 210 can detect synchronization signals, return a serial signal at intervals of the horizontal synchronization signal, and reproduce an image. The reproduction apparatus of this configuration can also steal image information.
An example of conventional techniques to prevent information leakage caused by leakage electromagnetic waves is shown in FIG. 5. This technique inserts a filter circuit 20 in an interface cable (such as a serial signal interface) connecting a computer 9 and a peripheral device 10 to each other, to suppress the levels of electromagnetic waves produced by the interface cable. This technique, however, is applicable only to a system employing an interface cable for connection. For example, an information system consisting of a desktop computer and a display includes an interface cable. A notebook personal computer having an integrated structure has no way to apply the technique. Since information itself is transmitted through the cable, it is impossible to surely remove the information from leakage electromagnetic waves. A filter provided for a casing having no shield is not always effective to reduce leakage electromagnetic waves.
FIG. 6 is a view explaining a conventional method of reducing leakage electromagnetic waves by providing a computer casing with an electromagnetic shield. This method may surely reduce electromagnetic waves. The shield, however, is expensive. In an information communication system, interface cables for information transmission must be arranged through the casing, and therefore, it is very difficult to establish a complete shield.
In these circumstances, there has been proposed a method of preventing information leakage by generating a false signal. For example, FIG. 7(a) shows a method of forming a mask signal serving as a false signal, to mask R, G, and B signals to be transmitted (refer to, for example, Japanese Unexamined Patent Application Publication No. Hei-5-151114). FIG. 8 shows a method of randomly delay R, G, and B signals (refer to, for example, Japanese Unexamined Patent Application Publication No. Hei-6-83296).
Radiated electromagnetic waves, however, have signal patterns close to differential components (di/dt) of R, G, and B current signals or differential components (dv/dt) of R, G, and B voltage signals. Accordingly, temporal changes of actual leakage electromagnetic waves will be as shown in FIG. 7(b). With the mask signal of FIG. 7(a), the actual leakage electromagnetic waves will provide a combinational waveform of FIG. 7(c). Despite the mask signal, the original signals are reproducible with a proper filter or integration circuit.
Generating a random false signal like the method of FIG. 8 deteriorates the S/N ratio of a receiver to make it difficult to reproduce signals. Signals, however, become reproducible with a reproduction apparatus 200 shown in FIG. 9. The reproduction apparatus 200B of FIG. 9 has a vertical synchronization signal generator 211 in addition to the reproduction apparatus 200A of FIG. 4. A synchronization signal generated by the vertical synchronization signal generator 211 is provided as a trigger signal to an A/D converter 209, so that a notebook personal computer 210 may compute time averages or carry out an autocorrelation process based on a signal string of several frames, to remove the random components.